1. Field of the Invention
The present invention relates to technology for extraction of part of an optical signal, more specifically to a method of extracting time-division-multiplexed optical signals with a series of optical pulses or a component of the signals, to an optical switch that utilizes the method and to an optical sampling oscilloscope that utilizes the optical switch.
2. Description of the Related Art
Increase in data volume and the need for long-distance communication in recent years have promoted a wide spread of devices and systems utilizing optical technology. A part of this technology, the optical switch, which extracts a part of an optical signal consisting of a series of optical pulses, is under research and development as a core element. The following methods are known as conventional technology for switching optical signals consisting of a series of optical pulses:
(1) A technology, which first converts received optical signals into electrical signals, switches the signal, and converts back to an optical signal using an optical modulator or laser. This system is referred to as OE/EO type.
(2) A technology, which switches a selected channel by synchronizing electrical signal with the channel, and operating optical modulators such as LiNbO3 modulator and EA (Electro-Absorption) modulator based on the synchronized signal.
(3) A technology, which carries out all switching processes by optical means without involving any electrical signals. To be more specific, the following methods are known as a part of this technology.
(3a) A method using a Mach-Zehnder Interferometer configured such that the phase difference between light passing through two waveguide arms is π.
(3b) A method utilizing nonlinear wave mixing such as four-wave mixing (FWM) and three wave mixing (TWM).
(3c) A technique, which utilizes the optical Kerr effect such as self phase modulation (SPM) or cross phase modulation (XPM).
(3d) A technique, using gain saturation effect such as cross gain modulation (XGM) and cross absorption modulation (XAM).
The following documents relate to the technology stated above. Non-patent documents 1 and 2 describe techniques to perform 3R regeneration without converting optical signal input into electrical signals. These 3R regeneration techniques yield regenerated signal output with a regular waveform, which are not influenced by jitter, by guiding input optical signal and clock signal regenerated from the optical signal to an optical gate circuit comprising highly-nonlinear fiber.    Patent document 1: Japanese published unexamined application No. H7-98464    Patent document 2: Japanese Patent No. 3494661    Non-patent document 1: S. Watanabe, R. Ludwig, F. Futami, C. Schubert, S. Ferber, C. Boener, C. Schmidt-Langhorst, J. Berger and H. G. Weber, “Ultrafast All-Optical 3R Regeneration”, IEICE Trans. Electron, Vol. E87-C, No. 7, July 2004    Non-patent document 2: S. Watanabe, “Signal Regeneration Technique in Optical Field”, Kogaku (Japanese Journal of Optics), Vol. 32, No. 1, pp. 10-15, 2003
The conventional technologies listed above have the following technical issues. The OE/EO type is up to 10 Gbps in practice, and research and development is proceeded to work toward practice use up to 40 Gbps. However, it requires dedicated electronic circuitry for every bit rate to be supported, and has a high-speed signal limit due to a limit in the operation speed of electronics. The above-mentioned technology (2) using electrical signals as driving signals or control signals has the same problem in terms of operation speed.
The above-mentioned technology (3) does not have a limited operation speed because it does not employ electrical signals, however adoption of high-speed signals more than 160 Gbps leads to issues such as losses of 10-30 dB on switching and a narrow range of wavelengths that can be switched. Decrease in switching efficiency causes a decrease in the optical S/N ratio and degradation of signal quality. Further, narrow operating bandwidth requires optical switches for each signal wavelength.